1. Field of the Invention
The present invention relates to a process for producing a silicon single crystal by the Czochralski method. The invention also relates to a heater which is intended for heating a crucible filled with silicon and is arranged below the crucible.
2. The Prior Art
For the production of silicon single crystals by the Czochralski method, solid silicon is melted in a quartz-glass crucible, also referred to hereafter as just "crucible". A seed crystal is then dipped into the melt and a single crystal is pulled from the melt. Since its incipience, the method has continued to be developed, and it is currently possible to pull single crystals having diameters in excess of 200 mm.
However, producing large single crystals raises a number of problems. One of these problems relates to the thermal and rheological conditions when the silicon is melted and when the single crystal is pulled. It is desirable to melt the silicon as quickly as possible and to obtain thermal and rheological conditions in the melt that are necessary for dislocation-free pulling of the single crystal. In order to produce large single crystals, correspondingly large volumes of silicon need to be melted in the crucible. As a rule, use is made of polycrystalline fragments or single crystals which contain dislocations and need to be remelted.
The heating energy required for the melting is usually provided by resistive heating elements which are arranged around the sides of the crucible and below the crucible. A device having heating elements of this type is, for example, disclosed in U.S. Pat. No. 5,360,599. A factor against melting the silicon quickly by heating the heating elements to temperatures which are much higher than the melting point of silicon is the fact that liquid silicon attacks the crucible if the wall of the crucible is overheated. There is a risk of particles being detached from the material of the crucible, reaching the crystallization front during the pulling of the single crystal, and causing dislocations. An excessive temperature gradient between the wall of the crucible and the crystallization front furthermore produces uncontrolled convection currents. These currents in turn lead to undesired local fluctuations in the temperature of the melt and the concentration of dopant and impurities in the melt.